Anisotropy of plasticity and structural transformations under uniaxial tension of iron crystallites

Abstract

Molecular dynamics simulation has been performed to investigate the onset and evolution of plasticity in nanosized bcc iron crystallites with different crystallographic orientation under uniaxial tension. Calculations have shown that the onset of plasticity in the iron crystallite is associated with the nucleation of either dislocations or twins. These defects nucleate and grow as a result of structural transformations that locally change the lattice type. In stretching along the [112¯] direction, plastic deformation begins mainly due to the formation and growth of a single twin, at the front of which the bcc → fcc → bcc transformation occurs. Dislocations nucleate in the interior of the twin as the twin grows. In stretching along the [11¯0] direction, plasticity results from the formation of a large number of twins in the crystallite. Regions with fcc and hcp lattice structure arise at the front of these twins. With tensile strain increase, the contribution of twinning to the crystallite accommodation decreases because twins transform into dislocations. The onset of plasticity in the sample stretched along the [1 1 1] direction has a pronounced dislocation character. New dislocations appear at existing dislocations through the formation of regions with fcc lattice. A large part of the dislocations escape to the free surfaces during stretching, leaving the crystallite with a cellular dislocation structure and a large number of vacancies. Tension both in the elastic region and above the yield point causes local bcc → fcc → bcc transformations in the crystallite. They always occur in regions with increased stresses. The formation of single fcc atoms in the elastic stretching region is of thermal fluctuation nature, and their average lifetime is one atomic vibration period. The lifetime of the fcc atoms at the twin front is determined by the front propagation rate and is about 5 atomic vibration periods.